Echoviruses are human picornaviruses which are responsible for a number of clinical syndromes including fatal disseminated infections in neonates and immunocompromised hosts as well as fever, and skin rashes in normal hosts. As a group the 32 serotypes of echovirus are the major cause of meningitis in the United States. Virus receptors are cell surface proteins which allow for virus binding to the host cell, the initial step in viral replication in mammalian cells. These proteins are often directly responsible for the tropism of the virus, and transfection of viral receptor cDNA can confer susceptibility upon cells which were not previously susceptible to infection. We have previously defined VLA/2 (an alpha2 beta1 integrin) as the receptor for echoviruses 1 and 8. Utilizing mouse-human chimeric proteins we have been able to map the echovirus binding site on VLA/2. Our recent data indicates that the I region of the alpha chain of VLA/2 is capable of binding echovirus and conferring susceptibility to infection to non- permissive cells. In this proposal we plan to map the binding site for echovirus type 1 at the amino acid level and prepare isolated I region protein to allow a detailed study of the virus host interaction with the long term goal of crystallizing both the virus and host receptor molecules. In addition we have now found that the glycosyl phosphatidyl inositol (GPI) linked complement regulatory protein Decay Accelerating Factor (DAF) is the receptor for a number of other echoviruses. This protein, which is composed of 4 short consensus repeat sequences attached to a GPI tail is highly expressed on the surface of endothelial cells and epithelial cells. It exists in a variety of soluble and membrane anchored forms in humans. We plan to define the DAF binding sites and investigate the role that this protein has in viral tropism and infectivity. Using DAF transfectants we will assess the role of the GPI tail in viral entry and replication. Finally our data indicate that several other echoviruses bind to an as yet undefined protein(s). Therefore we plan to finish our grouping of the echoviruses by defining the receptors for the remaining viruses to begin to understand echovirus tropism and pathogenesis.